While that setup looks like it might give acceptable results you do understand that it doesn't matter anyways, don't you, seeing as the only one that really wants to hear the cajon is the person beating on it.
I've done a few jazz shows now with a really talented guy on Cajon, a hi-hat and a ride cymbal. He has a snare-like attachment on the Cajon. Often plays with brushes. It actually sounds very good.
Regarding the internal mic, I have been able to dial that in and use it at a few festivals with success, but still with a SDC in front and SDCs on the cymbals.